Mono-ADP-ribosylation of arginine residues in proteins is a reversible modification that involves the following steps: (i) the transfer of an ADP-ribose moiety of nicotinamide adenine dinucleotide (NAD) to an arginine residue of a target protein, or to a free arginine residue, by an arginine-specific ADP-ribosyltransferase (ART) and (ii) the cleavage of the bond between ADP-ribose and arginine by an ADP-ribosylarginine hydrolase.
ARTs were first characterized in bacterial toxins, such as cholera toxin, diphtheria toxin, pertussis toxin, and pseudomonas exotoxin A. ADP-ribosyltransferase activity has since been identified in eukaryotic cells. The widespread expression of ARTs in eukaryotes, as well as in prokaryotes, suggests that the cycle of ADP-ribosylation/de-ADP-ribosylation of amino acid residues is widely involved in regulating protein activity. Moreover, specific ADP-ribose acceptors, such as arginine, may serve as regulatory switches. For example, ADP-ribosylation of a specific arginine residue in the dinitrogenase enzyme of the nitrogen-fixing bacteria Rhodospirillium rubrum has been shown to regulate the activity of this enzyme.
In eukaryotes, ART activity is linked to regulatory signals for critical cellular processes such as DNA repair and the maintenance of calcium or phosphorylation levels. In humans, altered cellular ADP-ribosylation levels have been linked to a number of diseases including lupus, diabetes and cancer, whereas bacterial toxins, such as cholera toxin and diphtheria toxin, catalyze the ADP-ribosylation of important metabolic or regulatory proteins in their human hosts.
The ability to identify specific amino acids that can be modified in order to regulate the activity of various proteins is critical in the development of medical treatments and therapies. Thus there is a need to identify additional stable protein modifications that have an effect on protein activity.